Walter W. Simpson

Experiments on a current‐toggled plasma‐opening switch

Plasma‐opening switches have been used to improve pulsed‐power wave shapes for over a decade. These switches have used the inertia of the plasma to hold the switch closed. This results in conflicting requirements when long hold‐off time and fast opening are required, and also results in variation in opening current due to variation in initial plasma fill. The current‐toggled plasma‐opening switch attempts to overcome these problems by using external magnetic fields rather than inertia to control the plasma conductor. Data will be presented showing several features of the operation of this switch. These data will be compared to models used to design the switch. The comparisons indicate that the mass can be measured approximately from fast coil data and that the slow coil flux does set the opening level of the current. They also indicate that the opening current is somewhat dependent upon plasma mass, and that the design of the field coils that provide the control fields must be done more carefully to provi...

Development and characterization of a Z-pinch-driven hohlraum high-yield inertial confinement fusion target concept

Initial experiments to study the Z-pinch-driven hohlraum high-yield inertial confinement fusion (ICF) concept of Hammer, Tabak, and Porter [Hammer et al., Phys. Plasmas 6, 2129 (1999)] are described. The relationship between measured pinch power, hohlraum temperature, and secondary hohlraum coupling (“hohlraum energetics”) is well understood from zero-dimensional semianalytic, and two-dimensional view factor and radiation magnetohydrodynamics models. These experiments have shown the highest x-ray powers coupled to any Z-pinch-driven secondary hohlraum (26±5 TW), indicating the concept could scale to fusion yields of >200 MJ. A novel, single-sided power feed, double-pinch driven secondary that meets the pinch simultaneity requirements for polar radiation symmetry has also been developed. This source will permit investigation of the pinch power balance and hohlraum geometry requirements for ICF relevant secondary radiation symmetry, leading to a capsule implosion capability on the Z accelerator [Spielman et...

Monochromatic x-ray backlighting of wire-array z-pinch plasmas using spherically bent quartz crystals

X-ray backlighting systems are being developed to diagnose z-pinch, inertial confinement fusion capsule, and complex hydrodynamics experiments on the 20 MA Sandia Z machine. The x-ray backlighter source is a laser-produced plasma created using the Z-Beamlet laser, a 2 TW, 2 kJ Nd:glass laser recently constructed at Sandia. As an alternative to point-projection radiography, we are investigating a different geometry [S. A. Pikuz et al., Rev. Sci. Instrum. 68, 740 (1997)] that uses spherically bent crystal mirrors to simultaneously obtain high spatial resolution and a narrow spectral bandwidth. Backlighting systems using the Si Heα line (1.865 keV) and the Mn Heα line (6.15 keV) are discussed. These systems are capable of spatial resolutions in the 5–10 μm range, a field of view as large as 5 mm by 20 mm, and a spectral bandwidth comparable to the width of the emission line used for backlighting.

Dynamics of a Z-pinch x-ray source for heating inertial-confinement-fusion relevant hohlraums to 120–160 eV

A z-pinch radiation source has been developed that generates 60 {+-} 20 KJ of x-rays with a peak power of 13 {+-} 4 TW through a 4-mm diameter axial aperture on the Z facility. The source has heated NIF (National Ignition Facility)-scale (6-mm diameter by 7-mm high) hohlraums to 122 {+-} 6 eV and reduced-scale (4-mm diameter by 4-mm high) hohlraums to 155 {+-} 8 eV -- providing environments suitable for indirect-drive ICF (Inertial Confinement Fusion) studies. Eulerian-RMHC (radiation-hydrodynamics code) simulations that take into account the development of the Rayleigh-Taylor instability in the r-z plane provide integrated calculations of the implosion, x-ray generation, and hohlraum heating, as well as estimates of wall motion and plasma fill within the hohlraums. Lagrangian-RMHC simulations suggest that the addition of a 6 mg/cm{sup 3} CH{sub 2} fill in the reduced-scale hohlraum decreases hohlraum inner-wall velocity by {approximately}40% with only a 3--5% decrease in peak temperature, in agreement with measurements.

X-ray imaging techniques on Z using the Z-Beamlet laser

The Z-Beamlet laser backlighter system at Sandia National Laboratories, which will be operational in 2001, will create a point or area source of high (or moderate) energy x rays behind a Z-accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] -driven target. In the former case with >2 kJ in up to four pulses of 80% of the 2ω energy in a ∼50-μm-diam focal spot, the resulting >4×1016 W/cm2 irradiances will generate ⩾8.950, 8.999 keV (zinc He-α, etc.) x rays. This high-energy source, as either a single point or four separate spots, will be used directly for four-frame point-projection x-ray imaging, and will attain spatial resolutions and signal-to-noise levels significantly better than presently possible on Z using existing methods. In combination with a ∼1 cm field of view, the technique will be well suited to the large, relatively opaque objects characteristic of Z experiments. This addition is anticipated to have a major impact upon the basic phy...

Symmetric inertial confinement fusion capsule implosions in a high-yield-scale double-Z-pinch-driven hohlraum on Z

Detailed radiation-hydrodynamics calculations indicate that the dual-63-MA Z-pinch high-yield (HY) 220-eV inertial confinement fusion concept [Phys. Plasmas 6, 2129 (1999)] may release 400 MJ of fusion yield, if pulse shaping, capsule preheat, and x-radiation drive uniformity can be acceptably controlled. Radiation symmetry is under detailed investigation in an advanced, 70-eV HY-scale scoping hohlraum [Phys. Rev. Lett. 88, 215004 (2002)] driven by the single 20-MA power feed of Sandia National Laboratories’ Z accelerator. The time-averaged polar radiation asymmetry, 〈ΔI〉/I, is inferred from direct distortion measurements of an imploding capsule’s limb-darkened (“backlit”) shell, via 6.7 keV point projection x-ray imaging. Thus far, 〈ΔI〉/I has been measured at the 3.0±1.4 (%) level, on the best shots, in hohlraums (cylindrical) with length/radius ratios L/R of 1.61 and 1.69, either side of a L/R=1.66 predicted optimum for a zeroed P2 Legendre mode. Simulations suggest that when scaled to 220 eV with zeroe...

Radiation symmetry control for inertial confinement fusion capsule implosions in double Z-pinch hohlraums on Z

The double Z-pinch hohlraum high-yield concept [Hammer et al., Phys. Plasmas 6, 2129 (1999)] utilizes two 63-MA Z pinches to heat separate primary hohlraums at either end of a secondary hohlraum containing the cryogenic fusion capsule. Recent experiments on the Z accelerator [Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories have developed an advanced single-sided power feed, double Z-pinch load to study radiation symmetry and pinch power balance using implosion capsules [Cuneo et al., Phys. Rev. Lett. 88, 215004 (2002)]. Point-projection x-ray imaging with the Z-Beamlet Laser mapped the trajectory and distortion of 2-mm diameter plastic ablator capsules. Using the backlit capsule distortion as a symmetry diagnostic, the ability to predictably tune symmetry at the <10% level in fluence by modifying the hohlraum geometry has been demonstrated. Systematic control of the time-integrated P2 Legendre mode asymmetry coefficient over a range of ±6% (±2% considering points nearest the...

Measurements of 4–10 keV x-ray production with the Z-Beamlet laser

In order to characterize the current backlighting capability of Sandia’s Z-Beamlet laser (ZBL) over a range of high photon energies, we measured the x-ray conversion efficiency of the focused 527 nm ZBL beam into 4–10 keV x rays from He-like emission of the elements Sc through Ge (excluding Ga). The measurements approximated ZBL’s nominal backlighting geometry and laser performance at Sandia’s Z soft x-ray facility by irradiating planar foil targets several microns thick rotated 30° to the laser beam axis with a 600 ps pulse at 1 TW. The focal spot diameter was about 150 μm. This study includes measurements of the K-shell x-ray spectrum, x-ray power, and x-ray spot size with an array of filtered high-bandwidth silicon diodes, a convex LiF crystal spectrometer, step wedge filtered x-ray film, and a filtered x-ray pinhole camera. We found agreement with previous work for comparable laser parameters and recorded decreasing conversion efficiency versus atomic number and He-like photon energy.

Measurement of the efficiency of gold transmission gratings in the 100 to 5000 eV photon energy range

Three x-ray spectrometers, each with a transmission grating dispersion element, are routinely used at the Z soft x-ray facility to measure the spectrum and temporal history of the absolute soft x-ray power emitted from z-pinch and hohlraum radiation sources. Our goal is to make these measurements within an accuracy of ±10%. We periodically characterize the efficiency of the gratings used in the spectrometers by using an electron-impact soft x-ray source, a monochromator, grazing-incidence mirrors, thin filters, and an x-ray charge-coupled device (CCD) detector. We measure the transmission efficiency of the gratings at many photon energies for several grating orders. For each grating, we calculate efficiency as a function of photon energy using published optical constants of gold and multiple-slit Fraunhofer diffraction theory and fit the calculation to the measurements using the physical parameters of the grating as variables. This article describes the measurement apparatus and calibration techniques, di...

Zero-dimensional energetics scaling models for z-pinch-driven hohlraums

Wire array z pinches on the Z accelerator provide the most intense laboratory source of soft X rays in the world. The unique combination of a near-Planckian radiation source with high X-ray production efficiency (10 to 15% wall plug), large X-ray powers and energies (>100 TW, ≥0.8 MJ in 6 ns to 7 ns), large characteristic hohlraum volumes (0.5 to >10cm 3 ), long pulse lengths (5 to 20 ns), and low capital cost ( 200 MJ yield) ICF capsules with adequate radiation symmetry and margin. The z-pinch-driven hohlraum approach of Hammer et al. (1999) may provide a conservative and robust solution to the requirements for high yield, and is currently being studied on the Z accelerator. This paper describes a multiple-region, 0-D hohlraum energetics model for z-pinch-driven hohlraums in four configurations. We observe consistency between the model and the measured X-ray powers and hohlraum wall temperatures to within ±20% in X-ray flux, for the four configurations. The scaling of pinch energy and radiation-driven anode-cathode gap closure with drive current are also examined.